1. Field of the Invention
The present invention relates to a method of controlling a DC/DC converter disposed between a first power device and a second power device for energizing switching devices with given duty ratios, a method of controlling a DC/DC converter apparatus incorporating such a DC/DC converter, and a method of controlling driving operation of an electric vehicle incorporating such a DC/DC converter apparatus.
2. Description of the Related Art
Heretofore, there has been proposed an electric vehicle wherein a travel electric motor is energized by a battery and a fuel cell, the fuel cell being connected to the travel electric motor through an inverter and the battery being connected to the fuel cell through a DC/DC converter which functions as a bidirectional voltage converter (see, Japanese Laid-Open Patent Publication No. 2007-159315).
A DC/DC converter apparatus which incorporates the DC/DC converter controls the voltage across the secondary side of the DC/DC converter (hereinafter referred to as “secondary voltage”), i.e., the voltage (inter-terminal voltage) of the fuel cell, and also controls electric currents flowing through the respective phases of the DC/DC converter so that the electric currents will be lower than an allowable current level depending on the temperature of each of the switching devices of the phases of the DC/DC converter.
Providing an accessory is connected to the battery, if the power cable interconnecting the battery and the DC/DC converter is disconnected, it is desirable to interrupt an ordinary process of controlling the secondary voltage (hereinafter referred to as “secondary voltage control mode”) and to control the voltage across the primary side of the DC/DC converter (hereinafter referred to as “primary voltage”), i.e., to control the voltage across the battery for protecting the accessory for normal operation (hereinafter referred to as “primary voltage control mode”) in order to keep the voltage applied to the accessory connected to the battery at an appropriate level.
In the electric vehicle, a fuse is connected to an output terminal of the battery. When an overcurrent flows from the battery, the fuse is immediately blown out to prevent the overcurrent from further flowing from the battery, thereby protecting the battery. If the fuse is blown out, then the battery is unable to supply electric power through the DC/DC converter and an inverter to the electric motor until the fuse is replaced with a new one. As a result, the electric vehicle cannot be driven in the meantime. Therefore, it is desirable to interrupt the secondary voltage control mode before the fuse is blown out, and to change to a control process for keeping the current flowing through the DC/DC converter below a given threshold level (hereinafter referred to as “current limiting mode”).
The operation modes of the DC/DC converter, i.e., the primary voltage control mode, the secondary voltage control mode, and the current limiting mode, have different variables to be controlled. Consequently, simply switching from one of the operation modes to another tends to cause voltage and current fluctuations (disturbances) such as temporary overshoots of the primary voltage, the secondary voltage, and the current upon switching between the operation modes.